Flat panel display technology mainly features a liquid crystal display (LCD), a projection display and a plasma display panel (PDP), which have captured the market in the field of TVs, and with the improvement in related technology, a field emission display (FED), an electroluminescent display (ELD), etc. are expected to occupy fields per characteristic. Today, the range of LCDs has expanded into a notebook, a personal computer monitor, a liquid crystal TV, an automobile, an airplane, etc., and LCDs account for approximately 80% of the flat panel market, and are globally booming currently because of a rapid increase in demand.
In a conventional LCD, liquid crystals and an electrode matrix are placed between a pair of absorptive optical films. In the LCD, the liquid crystal part is moved by an electrical field generated by applying a voltage to two electrodes, and has an optical state changed thereby. Such treatment enables “pixels” carrying information to display an image using polarized light in a specific direction. Because of this, the LCD includes a top-side optical film and a bottom-side optical film, inducing polarization.
In an optical film used in the LCD, efficiency of light utilization emitted from a backlight is not necessarily high. This is because 50% or more of light emitted from the backlight is absorbed by a bottom-side optical film (absorptive polarizing film). Therefore, to increase the light-use efficiency of the backlight in the LCD, a reflective polarizer is installed between an optical cavity and a liquid crystal assembly.
FIG. 1 is a diagram illustrating an optical principle of a conventional reflective polarizer. In detail, p-polarized light of light moving from an optical cavity to a liquid crystal assembly is set to be transferred to the liquid crystal assembly by passing through a reflective polarizer, and s-polarized light emitted from a reflective polarizer is reflected to the optical cavity, reflected on a diffusing reflective surface of the optical cavity while polarization direction of light is randomized, and then is sent back to the reflective polarizer, resulting in conversion into the p-polarized light that can pass through the polarizer of the liquid crystal assembly. Then, the p-polarized light is transferred to the liquid crystal assembly through the reflective polarizer.
Selective reflection of the s-polarized light and transmission of the p-polarized light for incident light of the reflective polarizer are achieved by a difference in refractive index between optical layers while one planar optical layer having an anisotropic refractive index and the other planar optical layer having an isotropic refractive index are mutually and alternately stacked, setting of optical thicknesses of the optical layers according to elongation of the stacked optical layers, and a change in refractive indexes of the optical layers.
That is, as light incident to the reflective polarizer passes through the optical layers, the reflection of the s-polarized light and the transmission of the p-polarized light are repeated, and therefore only the p-polarized light of the incident polarized light is transferred to the liquid crystal assembly. Meanwhile, the reflected s-polarized light is, as described above, reflected while a polarization state is randomized at the diffusing reflective surface of the optical cavity, and then is sent back to the reflective polarizer. Therefore, power waste as well as the loss of light generated from a light source may be reduced.
However, since such a conventional reflective polarizer was manufactured to have an optical thickness and a refractive index between the optical layers that enabled optimization for the selective reflection and transmission of the incident polarized light by alternately stacking planar isotropic optical layer and anisotropic optical layer, which had different refractive indexes, and elongating these optical layers, a process of manufacturing the reflective polarizer was complicated. Specifically, since, due to the planar structure of the optical layer of the reflective polarizer, the p-polarized light had to be separated from the s-polarized light, corresponding to a wide range of an incidence angle of the incident polarized light, the number of stacked optical layers was increased so much that a production cost was exponentially increased. Also, because of a structure in which the optical layers are excessively stacked, optical performance was degraded by optical loss.
FIG. 2 is a cross-sectional view of a conventional multilayer reflective polarizer (DBEF). In detail, the multilayer reflective polarizer has skin layers 9 and 10, which are formed on both surfaces of a base 8. The base 8 is divided into four groups 1, 2, 3 and 4, each group including approximately 200 layers by alternately stacking isotropic layers and anisotropic layers. Meanwhile, separate adhesive layers 5, 6 and 7 are formed between the four groups 1, 2, 3 and 4 forming the base 8 to bond them. Also, since each group has a very small thickness of more or less 200 layers, when these groups are individually coextruded, the groups may be damaged, and therefore probably have a protective layer (PBL) in many cases. In this case, the base became thicker, and a production cost was increased. Also, since a reflective polarizer included in a display panel has a limited base thickness for slimming, when an adhesive layer is formed on a base and/or a skin layer, the base is reduced by the thickness of the adhesive layer, and therefore there was a serious problem for improvement of optical properties. Furthermore, since the inside of the base, the base and the skin layer are bonded by the adhesive layer, an interlayer peeling phenomenon may be generated by an external force, long periods of time elapsed or a poor storage area. There also was a problem of an excessively high defect rate in the attachment of the adhesive layer, and destructive interference to a light source due to the formation of the adhesive layer.
When the skin layers 9 and 10 are formed on both surfaces of the base 8, separate adhesive layers 11 and 12 are formed between the base 8 and the skin layers 9 and 10 to bond them. When a conventional polycarbonate-based skin layer is integrated with a PEN-coPEN alternately-stacked base by coextrusion, peeling may occur due to the loss of compatibility, the risk of birefringence with respect to an elongation axis in an elongation process is high since a crystallization degree is within 15%. Accordingly, to apply a polycarbonate sheet for a non-elongation process, an adhesive layer had to be formed. As a result, due to the additional adhesive layer process, a yield is reduced by external impurities and process defects, and usually, to produce a non-elongated polycarbonate sheet of the skin layer, birefringence is caused by non-uniform shear stress generated in a winding process. For this reason, separate controls such as transformation of the molecular structure of a polymer and control of the velocity of an extrusion line are required, and thus productivity is degraded.
Simply describing a method of manufacturing the conventional multilayer reflective polarizer, four groups having different average optical thicknesses enabling formation of the base are separately coextruded, subjected to elongation, and bonded with an adhesive, resulting in manufacturing a base. This is because peeling is generated by elongating the base after the bonding with the adhesive. Afterward, a skin layer is bonded to both surfaces of the base. To form a multilayer structure, one group (209 layers) has to be formed in a process of forming a multilayer structure including forming a tetralayer structure by folding a bilayer structure, continuously folding the structure, and coextruding the resultant structure. Therefore, it was difficult to form a group in the multilayer structure using one process without a thickness change. As a result, after separate coextrusion, the four groups having different average optical thicknesses had to be bonded.
Since the above-described process was intermittently performed, it led to a considerable increase in production costs, and thus among all of the optical films included in a backlight unit, the resultant multilayer polarizer had the highest cost. Therefore, for the sake of cost reduction, there were serious problems of frequently introducing an LCD excluding a reflective polarizer even when the reduction in luminance was ensured.
Therefore, a reflective polarizer, not a multilayer reflective polarizer, in which dispersed bodies capable of achieving the function of a reflective polymerizer, by aligning birefringent polymers elongated in a lengthwise direction in a base, are dispersed has been suggested. FIG. 3 is a perspective view of a reflective polarizer 20 including rod-shaped polymers, in which birefringent polymers 22 elongated in a lengthwise direction in a base 21 are aligned in one direction. Accordingly, a light modulating effect is caused at a birefringent interface between the base 21 and a birefringent polymer 22 to perform the function of the reflective polarizer. However, compared with the above-described alternately-stacked reflective polarizer, it was difficult to reflect light over the entire visible wavelength range, and thus light modulation efficiency was overly decreased. Thus, to have transmittance and reflectivity similar to those of the alternately-stacked reflective polarizer, an excessive number of birefringent polymers 22 had to be disposed in the base. In detail, to manufacture a 32-inch wide display panel based on the vertical cross-section of the reflective polarizer, at least 1×108 round or elliptical birefringent polymers 22 having the lengthwise cross-sectional diameter of 0.1 to 0.3 μm have to be included in a base 21 having a width of 1580 mm and a height (thickness) of 400 μm or less to have optical properties similar to those of the above-described stack-type reflective polarizer. In this case, there were difficult problems in which the reflective polarizer had an excessive production cost and was manufactured by overly complicated equipment in a production facility, thus making it nearly impossible to be commercialized. Also, since it is difficult to form a birefringent polymer 22 included in the sheet with various optical thicknesses, it is difficult to reflect light over the entire visible range, and thus physical properties are degraded.
To overcome these obstacles, a technical idea including a base having a birefringent islands-in-the-sea fiber has been suggested. FIG. 4 is a cross-sectional view of the birefringent islands-in-the-sea fiber included in the base. Since the birefringent islands-in-the-sea fiber may have a light modulating effect generated at a light modulating interface between an island and a sea therein, even though a very large number of islands-in-the-sea fibers are not disposed, like the above-described birefringent polymer, optical properties may be achieved. However, the birefringent islands-in-the-sea fibers are fibers, and thus have problems of compatibility, handleability and cohesion with the base, which is a polymer. Further, as reflective polarization efficiency for an optical wavelength in the visible range is reduced by induction of light scattering because of a round shape, compared with the conventional products, the reflective polarizer has poor polarization characteristics and thus has a limitation in luminance improvement. In addition, in the islands-in-the-sea fibers, since pores were generated by the reduction of an island coupling phenomenon and the subdivision of a sea component region, the optical characteristics were degraded due to light leakage, that is, optical loss. Also, an organizational form of the fabric had a limitation in improvement in reflection and polarization characteristics because of the limitation of a layered configuration. Moreover, a dispersion-type reflective polarizer had a problem of bright line visibility because of the gap between layers and the space between dispersed bodies.